The invention relates to a catalyst for the purification of exhaust gases of internal combustion engines by means of oxidation and/or reduction. The catalysts can contain the platinum group metals; namely, platinum, palladium and rhodium individually or in combination. They can be used, depending on the composition, as reduction catalyst of a double-bed reactor (in this instance the exhaust gas is first conducted to a reduction catalyst and then after admixture of air is conveyed to an oxidation catalyst) or as multifunctional catalyst for the simultaneous conversion of the oxidizable and the reducible components of the exhaust gas.
Engine exhaust-gas catalysts, especially those with high cerium oxide content, can bind sulfur oxides under oxidizing exhaust-gas conditions. With the change to reducing exhaust gases during travel, the amounts of sulfur oxide bound to the catalysts in this manner can be partially released again as hydrogen sulfide which is highly undesirable.
Since engine exhaust gases can contain sulfur dioxide as a result of using fuels containing sulfur and because the exhaust-gas catalyst experiences both oxidizing as well as reducing conditions in operation, depending on the engine operating conditions, hydrogen sulfide can be emitted when using traditional auto exhaust-gas catalysts. The H.sub.2 S odor threshold can be briefly exceeded thereby in a noticeable fashion.
It has been known for sometime that catalysts containing nickel exhibit only very low hydrogen sulfide emissions; however, the use of nickel as a component in engine exhaust-gas catalysts is problematic because of its suspected carcinogenic action. It should therefore basically be avoided.
There is, therefore, a need based on environmental reasons for nickel-free engine exhaust-gas catalysts with a reduced tendency to emit hydrogen sulfide.
The present invention starts essentially with the general teaching of DE patent No. 29 07 106 (to which U.S. Pat. No. 4,294,726 corresponds) for the formulation of exhaust-gas purification catalysts and expands this teaching as concerns the potential elements of the oxide layer and their combinations by a new component with which it is possible to eliminate the above-mentioned deficiency to a large extent.